1. Field of the Invention
The present invention relates to a fuel injection valve, which is installed to an internal combustion engine and injects fuel through an injection hole thereof.
2. Description of Related Art
In order to accurately control an output torque and an emission state of an internal combustion engine, it is important to accurately control an injection state of a fuel injection valve (e.g., start timing of fuel injection at the fuel injection valve and the amount of fuel injected from the fuel injection valve). In view of the above point, there has been proposed a technique for sensing an actual injection state by sensing a pressure of fuel, which changes in response to fuel injection. For example, the actual start timing of the fuel injection may be sensed by sensing the start timing of decreasing of the fuel pressure caused by the start of the fuel injection, and the actual end timing of the fuel injection may be sensed by sensing the stop timing of increasing of the fuel pressure caused by the termination of the fuel injection (see, for example, Japanese Unexamined Patent Publication No. 2008-144749A corresponding to US 2008/0228374A1).
When a fuel pressure sensor (rail pressure sensor), which is directly installed to a common rail (accumulator), is used to sense the change in the fuel pressure, accurate measurement of the change in the fuel pressure is difficult since the change in the fuel pressure caused by the fuel injection is buffered in the common rail. In the case of the invention recited in Japanese Unexamined Patent Publication No. 2008-144749A, the fuel pressure sensor is installed to the fuel injection valve to sense the change in the fuel pressure caused by the fuel injection before the change in the fuel pressure is buffered in the common rail.
In the above fuel injection valve, a body has a high pressure passage, which conducts high pressure fuel to the injection hole. The body receives a needle and an actuator. The needle is reciprocated away from or toward the injection hole to open or close the injection hole, and the actuator drives the needle. The inventors of the present application have previously proposed to install a fuel pressure sensor, which is constructed in the following manner, to the body. Specifically, the fuel pressure sensor includes a flexure element, a sensor element and a signal processing circuit. The flexure element is installed to the body and is adapted to be resiliently deformed upon application of the pressure of the high pressure fuel to the flexure element. The sensor element converts the strain, which is generated in the flexure element, into a corresponding electrical signal. The signal processing circuit performs, for example, an amplifying operation, which amplifies the measurement signal outputted from the sensor element.
Prior to shipment of the injector to a market, various tests and inspections need to be performed on the fuel pressure sensor. These tests and inspections will be described below.
A thermal expansion deformation of the flexure element is increased when the fuel temperature is increased. Therefore, the output value of the fuel pressure sensor (i.e., the sensor output value, which is outputted from the signal processing circuit) is drifted. Thereby, the fuel pressure needs to be computed based on the sensor output value in view of the amount of the temperature drift discussed above. The amount of the temperature drift may be a flexure element specific value, which may vary from one flexure element to another flexure element. Therefore, the amount of the temperature drift needs to be obtained in advance through experiments (temperature characteristic test) before shipment of the fuel injection valve to the market.
Thereby, in an assembled state, in which the flexure element, the sensor element and the signal processing circuit are installed to the body, fuel, which is under a test temperature and a test pressure, is supplied to the high pressure passage of the body to apply the pressure of the fuel to the flexure element. The amount of the temperature drift for this specific test temperature is obtained based on the sensor output value, the test pressure and test temperature of the fuel at this test time. Furthermore, an abnormality inspection of the fuel pressure sensor is performed by checking whether the sensor output value, which is obtained for the specific test pressure, is out of a normal range.
In the installed state, in which the flexure element is installed to the body, the temperature of the flexure element and the temperature of the body need to be stabilized to the test temperature. However, a thermal mass (also called thermal capacitance or heat capacity) of the body is relatively large. Therefore, an extra time is required to stabilize the temperature of the body to the test temperature. Furthermore, when the abnormality is detected in the abnormality inspection, which is performed on the fuel injection valve in the assembled state where the fuel pressure sensor is installed to the body, the entire fuel injection valve needs to be handled as an abnormal product. Thereby, it causes a reduction in the manufacturing yield of the fuel injection valve.